RPN

(define (list-head xs i S)
  (if (= i 0) S (list-head (cdr xs) (- i 1) (append S (list (car xs))))))

(define (priority a)
  (if (equal? a '*) 2 (if (equal? a '/)
                           3
                           (if (equal? a '+)
                                0
                                (if (equal? a '-)
                                    0
                                    100)))))

(define (operator? a)
  (or (equal? a '<) (< (priority a) 100)))

(define (delete< xs S)
  (if (null? xs) S (if (equal? (car xs) '<)
                       (delete< (cdr xs) S)
                       (delete< (cdr xs) (append S (list (car xs)))))))

(define (ejection x stack)
   (if (null? stack) 0 (if (or (operator? x) (equal? x '>))
                           (let ((k (car stack)))
                           (if (equal? k '<)
                               (length stack)
                               (if (equal? x '>)
                                   (ejection x (cdr stack))
                                   (if (<= (priority x) (priority k))
                                       (ejection x (cdr stack))
                                        (length stack)))))
                        (length stack))))

(define (rpn xs)
  (define (iter xs out stack)
    (if (null? xs)
        (append out (delete< stack '()))
        (let* ((a (car xs))
           (l (cdr xs))
           (k (if (and (not (null? stack)) (equal? a (car stack))) (length stack) (ejection a stack)))
           (nstek (list-tail stack (- (length stack) k)))
           (nstek (if (equal? a '>) (cdr nstek) nstek))
           (newstek (if (operator? a) (cons a nstek) nstek))
           (stack+ (list-head stack (- (length stack) k) '())))
          (iter l (append out (if (and (not (equal? a '>)) (not (operator? a))) (list a) '()) stack+) newstek))))
  (iter xs '() '()))